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Colorblind human fruit-lovers face a problem at the grocery store: How can you tell if a banana is ripe if you can’t see green properly? Bernhard Kräutler and his team at the University of Innsbruck may have come up with an answer. Ripe bananas fluoresce under UV light. “Surprisingly, the blue luminescence of bananas apparently has been entirely overlooked. Most humans would even consider the idea of a blue banana to be unappetizing,” Kräutler writes in a paper on the research (Angew. Chem. Int. Ed. 2008, DOI: 10.1002/anie.200803189). As chlorophyll in banana peels and leaves breaks down during ripening, it produces a set of blue fluorescent compounds, which peak in concentration just as the fruit inside is peaks in ripeness. As the fruit continues to mature, the compounds break down further into nonfluorescent molecules. Kräutler suggests in the paper that the fluorescence may have evolved as a way to signal to banana-eating animals, many of which can see further into the UV than humans can, that the fruit was perfect to eat.